1. Field of the Invention
The present invention relates to the filtration of filterable liquid media under conditions of non-steady tangential flow for purposes of liquidmedium separation operations, especially filtration, microfiltration, ultrafiltration or reverse osmosis.
2. Description of the Prior Art
It is known to this art to conduct filtration operations underconditions of tangential flow with a view to separating the constituents of a liquid medium by reason of, in particular, their size, their shape and their physical characteristics, utilizing pressure as the driving force.
According to this technique, the liquid medium to be treated is circulated tangentially over one of the face surfaces of a planar or tubular organic or inorganic porous membrane having particular adapted porosity.
Under the influence of a pressure differential between the two face surfaces of the membrane, at least one of the constituents of the fluid is selectively transported through the membrane. This liquid fraction thus recovered is designated the permeate. The remainder of the liquid is designated the retentate.
These organic or inorganic membranes typically comprise, on the one hand, a porous support, the principal function of which is to impart mechanical properties to the overall membrane and, on the other, a very thin permeable separating layer providing the separation and including pores having diameters adapted to the liquids to be treated.
In industrial filtration devices, the fluid to be treated is transferred into at least one filtration module which generally comprises a plurality of planar or tubular membranes arrayed side-by-side and assembled within an enclosure.
This module comprises at least one inlet for a fluid to be treated, one outlet for the retentate and one outlet for the permeate.
Planar filtration modules are, for example, described in U.S. Pat. No. 4,415,447.
Multi-tube filtration modules are described, for example, in FR-A 2,228,518, U.S. Pat. No. 4,341,631 and EP-A 025,349.
These multi-tube ultrafiltration modules may also be in the form of a perforated monolithic assembly as, for example, described in FR-A 2,585,965 and U.S. Pat. No. 4,069,157.
The modules are situated in a loop providing continuous circulation of the liquid medium to be treated, designated a circulation loop, comprising means (generally a pump) for circulating the fluid. The loop is connected to the inlet and outlet orifices of the module. The loop also comprises an inlet for fresh liquid to be treated and an outlet for concentrated liquid.
The ratio of the inlet flowrate to the outlet rate defines the concentration factor.
A principal drawback of liquid-medium tangential filtration using such a circulation loop is the occurrence, after a certain period of operating time, of at least one phenomenon limiting the effectiveness of the filtration, more fully described below and which is responsible for the "fouling" of the membrane.
As intended herein, "fouling" of the separating layer of the membrane therefore connotes the occurrence of at least one of these phenomena limiting the effectiveness of the filtration. This fouling constitutes the fundamental problem to be solved in respect of tangential filtration, although this type of filtration itself represents a significant improvement in this art. The first limiting phenomenon which is manifested over time is the establishment of an adsorption layer at the surface of the separating layer of the membrane, including the volume within the pores.
The importance of this layer is related to the chemical nature of the membrane, to the fluid to be treated, to its solutes and also to the specific surface area of the membrane. The adsorption will be greater as the pores become narrower and is especially prevalent in ultrafiltration.
The second limiting phenomenon is the actual closing of the pores via blockage by means of particles lodged within the pores which results in a reduction in the number of pores and/or a decrease in their average radius.
This closing of the pores may be prevented by a chemical washing. Such a washing is effective, but it obviously requires the shut-down of the installation and a lengthy and costly handling process.
Only in certain cases is it possible to unplug the pores by simple reversal of the direction of flow of the permeation. However, such a reversal, despite the increase in permeability, results in a loss of time and of permeate, which considerably limits its efficiency.
Another phenomenon exists which limits the efficiency of the filtration and which entails the creation of a deposit of particles at the surface of the separating layer of the membrane.
Indeed, the establishment of a deposit of particles is directly related to the liquid to be filtered and only appears in the event that the liquid to be filtered contains sufficiently large and numerous particles to form such deposit, generally visible to the naked eye. Furthermore, in the instance of solutions, another limiting phenomenon exists entailing the adsorption of all of the precipitated solutes on the membrane, rendered insoluble by reason of the increase in their concentration at the surface of the membrane, by the variations in pH and by the mechanical denaturing of the solutes by the pumps.
A final limiting phenomenon is the polarization layer, the establishment of which is associated with filtration of the solutions. This polarization layer is constituted by an overconcentration of the solute at the surface of the separating layer. It is destroyed by simple diffusion when the effects of convection tend to disappear. Its thickness, which is very small, is on the order of a few micrometers.
As indicated above, the known means for eliminating, at least temporarily, such fouling is chemical washing. In order to reduce the fouling, the known means entail reversing the direction of the permeation as indicated above and increasing the tangential flowrate of fluid to be treated.
Another known means for reducing fouling is to conduct a pulsed flow of the fluid within the module. Thus, V. Millisic et al ["Anti-fouling Technique in Cross-Flow Microfiltration", IVth World Filtration Congress, Ostend, Belgium (April 1986)] pulse a tangential flowrate of liquid by opening and closing a solenoid valve situated along the circulation loop upstream of the module. The pulses thus generated enable the fouling to be substantially reduced.
However, the circulation loop is subjected to significant water pounding, thus initiating a very great pressure increase in the membrane which indeed promotes such "fouling."
It too is known to generate pulses by means of adapted metering pumps or by the agency of a piston mounted in the loop that generates pulses by suction and delivery of a precise volume of liquid. In the case of using such piston, the pulses are in addition to the average flowrate of the loop. On the other hand, in the case of the metering pump, only the flowrate fluctuates about a zero average value.
All of these known means for generating pulses are not adapted to industrial scale systems which employ circulation flowrates on the order of several hundreds of m.sup.3 /h.
Moreover, these means/devices generate abrupt pressure variations which may damage the membranes and the solute and may promote redepositing the particles previously stripped off from within the pores of the membranes.